1. Field of the Invention
The present invention relates to a DNS (Domain Name System) server, a DHCP (Dynamic Host Configuration Protocol) server, a terminal and a communication system of an Internet capable of supporting a large number of terminals by means of a small number of IP addresses by dynamically assigning a limited number of IP addresses only to now communicating terminals.
2. Description of the Related Art
Among conventional Internets, as shown in FIG. 11, is a system for connecting a DNS server 50 to a plurality of terminals (personal computer terminals) 52a, 52b, 52c, - - - , 52n through a dedicated line or LAN/MAN 51. Usually, in the DNS server 50 are stored a look-up table for storing FQDNs (fully qualified domain name-host name) as shown in FIG. 12A and a reverse look-up table as shown in FIG. 12B. When a inquiry about a FQDN is made of a DNS server from a terminal (for example, 52a) connected to the DNS server 50 or an external terminal connected to the Internet in the case where the terminal is to be connected to a terminal corresponding to the FQDN, the DNS server 50 finds an IP address from the FQDN with reference to the look-up table, whereby the terminal is connected to the terminal corresponding to the FQDN. In some case, the DNS server 50 finds a FQDN from the IP address with reference to the reverse look-up table.
In the conventional system described above, there is presented a problem that each of the terminals 52a, 52b, 52c, - - - , 52n needs to have an address, which leads to the depletion of the IP addresses managed by the DNS server 50.
In order to solve this problem, for example, a system employing the DHCP server 60 shown in FIG. 13 is proposed. The DHCP server 60 has a function of assigning an IP address and is connected to, for example, a public exchange network 61 through 10 lines and the public exchange network 61 is connected to, for example, 100 terminals 62a to 62n. Usually, each of the terminals 62a to 62n is not assigned an IP addresses and when the terminal sends IP communications, it requires the DHCP server 60 to assign an IP address thereto and is assigned the IP address by the DHCP server 60.
For example, when a terminal 62a is going to conduct the IP communications, the DHCP server 60 assigns an IP address to the terminal 62a. As a result, the terminal 62a can start conducting the IP communications with the other terminals. When the terminal does not conduct the IP communications even after a predetermined time elapses after the completion of this communications, the terminal 62a returns the IP address to the DHCP server 60 and returns to the initial state where it has no IP address.
According to this conventional system, it is essential only that the DHCP server 60 assigns the IP address to the terminal which requires the IP communications every time it requires the IP communications, and hence the DHCP server 60 can manage the terminals 62a to 62n with a small number of IP addresses. Also, this can save the number of ports of the DHCP server 60.
In the conventional system described above, however, an IP address is not previously assigned to each of the terminals 62a to 62n, and hence the terminals 62a to 62n can not receive communication. In other words, the conventional system has a problem that it is designed specifically for sending communication.